The background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
Ethanol is currently receiving a great deal of interest as a renewable source of alternative transport fuel, with over 23 billion gallons (87 billion liters) of ethanol produced for fuel purposes in 2014 worldwide. Ethanol is typically derived via a fermentation process (for example, corn ethanol fermentations) and then concentrated via distillation and molecular sieves to produce a fuel grade product. Unfortunately, the formation of ethanol/water azeotropes for which the composition of the liquid and vapor are identical complicates recovery of high purity ethanol. As a result, removal of ethanol from fermentation broths to produce high purity ethanol suitable for use as a fuel or in fuel mixtures frequently requires processing through energy-intensive distillation steps that include application of energy-intensive processes that specifically remove water from azeotropic mixtures. These additional steps significantly impact the costs of producing fuel-grade ethanol, lower potential greenhouse gas reductions, and cast doubts on the sustainability of ethanol as a renewable fuel.
There are a number of shortcomings to the use of ethanol as a fuel. For example, significant adaptation of internal combustion engines is necessary to permit them to use ethanol as a fuel. Another factor is that ethanol is not truly fungible with conventional hydrocarbon fuels. For example, current infrastructures do not support transportation of ethanol via pipelines, but rather using tanker trucks and trains. Ethanol also has two-thirds the energy density of gasoline, which results in up to 50% more ethanol being needed to travel the same distance as gasoline. Because of differences in the properties of gasoline and ethanol most current vehicles are not warranted to use ethanol/gasoline blends containing more than 10% ethanol, while the existing infrastructure is limited to using up to 85% ethanol in gasoline (i.e. E85 fuel blend). The lower energy density and hygroscopic nature of ethanol prevent its use in aircraft that look to maximize energy content per mass of fuel and minimize water retention in the fuel. In addition, ethanol is not well suited for use in diesel engines in heavy-duty vehicles.
In addition to providing low energy density, production of fuel-grade ethanol from renewable sources (such as fermentation products) has relatively high energy requirements. Ethanol feedstocks provided from renewable sources typically has a high water content, which must be removed prior to use as fuel. At large scales this is typically achieved using one or more distillation processes, which have significant heat requirements. The limitations of ethanol distillation due to the formation of ethanol:water azeotropes necessitate the use of additional steps, such as the application of molecular sieves, to produce ethanol that is sufficiently anhydrous for fuel use. The regeneration of such molecular sieve materials constitutes an additional energy expenditure.
In addition to high energy costs the production of fuel grade ethanol from renewable sources also consumes considerable fresh water. While approximately 96% of the corn currently used for ethanol production is grown without irrigation (see Aden, A. “Water Usage for Current and Future Ethanol Production”, Southwest Hydrology, September/October 2007, pp: 22-23) and ethanol plants generally recover much of their processing water, significant water consumption occurs in boiler systems and cooling towers. Estimates are that between 3 and 4 gallons of water are consumed for every gallon of ethanol produced from fermentation of corn. All publications identified herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply. Water consumption is greater in biochemical conversion of cellulosic feedstocks to ethanol, averaging approximately 6 gallons of water for every gallon of ethanol produced. Water consumption for thermochemical conversion of cellulosic biomass to ethanol averages 1.9 gallons of water per gallon of ethanol. Most of this fresh water is sourced from groundwater.
Thus, there remains a need for systems and methods that reduce the energy and water requirements of processes producing alcohol fuels from the output of fermentation processes.